This invention relates generally to video systems and, in particular, to a technique for refreshing a dynamic memory that is used to drive a video display.
Typically, video image information is stored in the system in the form of digitized data that can be read out of the memory in a row by row mode to drive the video display in a line by line sequence. If static memories are used to drive the video display, they are capable of holding the data for the entire time period required to scan a full field. A typical dynamic memory, on the other hand, generally has to be refreshed about nine times during a conventional video field. Static memories, however, are relatively costly and space consuming. A dynamic memory, on the other hand, represents a cost attractive means for storing image data, requires less power to operate and is suitable for higher density construction, that is, more cells per unit area when compared to a static memory of the same capacity.
Many U.S. patents describe various schemes for refreshing dynamic memories. These patents that are known to applicants are:
______________________________________ 3,684,897 4,040,122 4,232,376 3,691,536 4,079,462 4,293,931 3,737,879 4,203,159 4,293,932 3,729,722 4,207,618 4,296,480 3,790,961 4,249,247 4,328,566 ______________________________________
None of these patents, however, describes a refreshing system that is especially adapted to the scanning rate of a standard video display wherein each field is scanned in slightly over sixteen milliseconds. It should be further noted that most of these prior art refreshing devices require a separate refresh address counter to carry out the refreshing function and another level of multiplexing, which increases the cost of the equipment and the size and complexity of the system.